Recreational Clams in the Coos Estuary Summary: Butter and Gaper Clam Populations in Some Areas of Lower Coos Estuary
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Recreational Clams in the Coos Estuary Summary: Butter and gaper clam populations in some areas of lower Coos estuary and South Slough have increased Butter clam Gaper since a similar assessment in the clam 1970s. Native littleneck clams and cockle populations have generally fallen since the 1970s in the areas sampled. Cockle Littleneck clam Population shifts of any estuarine-dependant animals should be cause for further investigation to understand potential causes of change. Source: ODFW 2014 Figure 1. 2009 ODFW SEACOR study areas located in the South Slough (SS) and Lower Bay (LB) subsystems. What’s happening? Recreational clamming is an important part of our area’s quality of life. To improve understanding of clam population size and distribution in the Coos estuary, Oregon Department of Fish and Wildlife’s (ODFW) Shellfish and Estuarine Assessment of Coastal method (DAM) at three study sites: South Oregon (SEACOR) project documented in Slough, Clam Island and Pigeon Point. 2008-09 the status of four recreationally Rapid assessment method (RAM) data from important bay clam species in the lower Coos other sites (Empire, Airport and North Spit) estuary- South Slough (SS) and Lower Bay were deemed preliminary data only (see (LB) subsystems (see Figure 1): gaper clams Background). Italicized text below indicates (Tresus capax); butter clams (Saxidomus sections quoted directly from the ODFW gigantea); native littleneck clams (Leukoma (2014) report unless otherwise noted. staminea); and cockles (Clinocardium nuttallii). SEACOR Study Results Project scientists estimated the distribution Clam beds were found in all areas studied by and abundance of each species within the SEACOR team. The greatest numbers of the SEACOR study sites and described the clam beds were found at the Clam Island and habitat requirements for each. The results Pigeon Point study sites (both in the Lower of this work are summarized in Table 1 Bay subsystem)(Figure 1). Overall, both gaper (ODFW 2014). The ODFW report summarizes clams and butter clams were found in much results from their detailed assessment greater densities than cockles and littleneck clams at all study sites (Table 1). Gaper clam Table 1. Mean clam density data per meter squared densities were the highest at the Clam Island for three SEACOR study areas in the lower Coos estuary. ODFW 2014. study site (Figure 2, Table 1) and butter clam densities were high at both Clam Island and Pigeon Point (Figure 3, Table 1), which are both located in the lower Coos estuary subsystem. Cockles were found in relatively high densities only in the South Slough study area (Figure 4, Table 1), while native littleneck clams were found only in low densities at all three sites, the most dense being Pigeon Point (Figure 5, Table 1). The low densities of littleneck clams in the 2009 SEACOR study contrasts with earlier surveys conducted in the 1970s when native littleneck clams were found to be reasonably abundant in the Coos estuary. Figure 2. Gaper clam abundance and distribution in SEACOR Figure 3. Butter clam abundance and distribution in SEACOR study areas Data from Empire, Airport and North Spit sites are study areas. Data from Empire, Airport and North Spit sites are considered preliminary only. Data: ODFW 2014. considered preliminary only. Data: ODFW 2014. Figure 4. Cockle abundance and distribution in the SEACOR Figure 5. Littleneck clam abundance and distribution in the study areas. Data from Empire, Airport and North Spit sites are SEACOR study areas. Data from Empire, Airport and North Spit considered preliminary only. Data: ODFW 2014. sites are considered preliminary only. Data: ODFW 2014. Additional detail, organized by study sites, is found in ODFW’s SEACOR final report (2014): Clam Island supported high densities and diverse assemblages of bay clams (Figure 6). Butter clams were abundant across all tidal strata at Clam Island (site mean=4.6 clams/m2). Cockles were least abundant at Clam Island, compared to the other tide flats surveyed by DAM, and were most abundant in the mid-intertidal (mean = 0.6 clams/m2). Gaper clams exhibited the greatest densi- ties at Clam Island (mean=19.9 clams/m2) and were most dense in the low intertidal (mean=45.6 clams/m2). Native littleneck clams exhibited low densities at all three sites 2 2 Figure 6. Clam distribution and abundance (clams/m ) at the (mean=0.7 clams/m ), including Clam Island SEACOR Clam Island study site. Note the difference in scale for (mean=0.4 clams/m2). each clam species. Data are from DAM surveys only. Data and figure: ODFW 2014. Pigeon Point exhibited the highest densities of butter clams (Figure 7), particularly at high tidal elevations (H, mean=15.0 clams/m2) in sandy areas commonly referred to as “butter bars.” Cockles were not very abundant at Pigeon Point (mean=1.0 clams/m2), and were most prevalent in the mid tidal stratum at Pigeon Point (mean=2.2 clams/m2). Gapers were less abundant at Pigeon Point compared to Clam Island (mean=3.1 clams/m2). Low tid- al elevations harbored the greatest densities of gapers across all sites (mean=19.3 clams/ m2) including Pigeon Point (mean 8.2 clams/ m2). Littleneck clams exhibited the highest densities at Pigeon Point (mean=1.4 clams/ m2), where they showed little difference in density among tidal strata. Figure 7. Clam distribution and abundance (clams/m2) at the SEACOR Pigeon Point study site. Note the difference in scale for each clam species. Data are from DAM surveys only. Data and figure: ODFW 2014. South Slough exhibited the lowest density of butter clams (mean=2.1 clams/m2), but they were still abundant compared to other clams such as cockles and littlenecks (Figure 8). The highest densities of cockles occurred at South Slough (mean=1.5 clams/m2), particularly in high and low tidal elevations (mean=2.0 clams/m2 and 1.6 clams/m2, respectively). The largest cockles were also collected at South Slough (mean shell length=70.2 mm). Gapers were least abundant at South Slough (mean=1.8 clams/m2) among the three sites sampled by DAM. Generally, gaper clam length increased with decreasing tidal stra- tum except at South Slough, which lacked smaller gaper clams. Littleneck clams were least abundant at South Slough (mean=0.3 clams/m2)(Figure 8) and were entirely absent at many waypoints surveyed by DAM. Figure 8. Clam distribution and abundance (clams/m2) at the SEACOR South Slough study site. The scale for each clam species is the same. Data are from DAM surveys only. Data and figure: ODFW 2014. Clam Bed Attributes Clam bed attributes including bed type, eelgrass cover, sediment type and intertidal • Cockles and littleneck clams were most elevation were measured by ODFW scientists frequently found in oxygenated clam bed and summarized for the Coos estuary in Fig- sediments. ures 9-12. They determined through a series • The presence of gaper clams was neg- of regression analyses the following: atively correlated with sediment tem- • The presensence of algae on clam beds perature “at depth” (the depth in the was negatively correlated with the pres- sediment where those clams are normally nce of butter clams- these clams were found). So, the higher the sediment tem- almost always found in non-vegetated peratures were, the fewer gaper clams sand flats. were found. Gapers were found in great- est abundances in low intertidal flats. • The presensence of algae in clam beds was positively correlated with the pres- • The presence of native eelgrass beds nce of cockles and with the presence of (Zostera marina) was positively correlated gaper clams. with the presence of littleneck clams (in the few areas they were found). Figure 10. Major sediment type in the SEACOR study areas. Data: ODFW 2014. Figure 9. Intertidal bed type in the SEACOR study areas. Data: ODFW 2014. Figure 11. Eelgrass cover in the SEACOR study areas. Data: Figure 12. Tideal height range in the SEACOR study areas. Data: ODFW 2014. ODFW 2014. Why is it happening? The status of clam populations is a function of a variety of environmental conditions includ- ing but not limited to bathymetry (shape of the estuary bottom), human activities (e.g., commercial oyster culture, dredging of the commercial shipping channel), and the health of other key benthic and pelagic communities such as eel grass beds and crustaceans (Carl- ton et al. 1991; Yocom and Edge 1929). Furthermore, the status of local environmen- tal conditions is determined by many chang- ing elements associated with both natural and human-induced changes. For example, the effects of major storms or the establish- ment of an oyster culture system have the po- Figure 13. Locations of all previously conducted clam surveys tential to change the bathymetry of an area in Coos Bay via sedimentation or scouring of the estuary bottom. These same actions may also affect the health of nearby eelgrass communities Detailed comparisons between studies (Carlton et al. 1991). Determining the effects conducted during different time periods are of these activities is a complicated process made difficult by gaps in the data. For exam- requiring collection of quanitative data over ple, only two published clam surveys of Pony multiple time periods. Slough have been completed, with the most recent data coming from the 1970s (Macdon- Past Studies ald 1967; Bottom et al. 1979). Additionally, Previously conducted clam research in the some areas such as the Upper Bay subsystem, Coos estuary provides information that can for which the Marriage study (1958) rep- be used to characterize trends in local clam resents the most current data, have not been populations. The research presented here surveyed for many years. Methodological represents 80 years of published clam survey differences between studies further compli- data.